Battery-operated small electrical devices are typically charged at an external charging station. Contactless charging stations that inductively transmit electric energy from the charging station to the device are known in the art. For this, an alternating magnetic field is generated in the charging station by an oscillator that includes a coil element and a capacitor element, wherein the coil element simultaneously forms the primary coil of an inductive transformer and the secondary coil of the transformer is arranged in the device to be charged. The charging station is therefore conventionally designated as the primary side and the device to be charged is designated as the secondary side. Such a charging station in which the oscillator is operated with a stabilized voltage or, respectively, oscillates with a uniform amplitude is known from JP 6-54454 A.
Modern charging stations typically have three operating states. The first state is the operating mode in which the secondary side continuously draws power, for example, to operate the device or to charge a cell installed in the device. The second state is the simple standby mode in which the device is not located in the charging station, thus in which no power whatsoever is drawn. The third state is what is known as the extended standby mode in which the device is located in the charging station but only requires power intermittently, for example, because—although the cell is fully charged—it must occasionally be recharged to compensate for the self-discharge or for the device's own power consumption. In the latter instance, the charging station should switch back and forth between the simple standby mode and the operating mode as needed. The respective operating state of the charging station (primary side) is thus determined by the power demand of the small electrical device (secondary side).
It is known to detect the power demand of the secondary side directly at the secondary side, to transfer corresponding information to the primary side and to adjust the oscillator—meaning, for example, the base emitter voltage of a transistor operating in the oscillator—accordingly. This solution is quite complicated because transmission means for the information from the secondary to the primary side are required. Alternatively, the power demand of the secondary could be determined by measuring the power consumption of the oscillator (at the primary side) and controlling the oscillator accordingly. However, both variants are poorly suited to the setting of multiple operating states because the power consumption of the charging station is only slightly affected by the power consumption of the device due to the typically weak coupling between the primary and secondary side of the transformer.
As such, there is a need to specify a circuit arrangement for inductive power transfer from a primary side to a secondary side that may establish the power demand of the secondary side at the primary side in a simple manner.